1. Field of the Invention
The present invention relates to a camera having an electronic flash equipment for emitting flash light of the controlled quantity of light in synchronism with a shutter operation.
2. Description of the Related Art
Hitherto, there is well known a camera having an electronic flash equipment in which in case of a shortage of the quantity of light from a camera subject, flash light of the controlled quantity of light is emitted in synchronism with a shutter operation to perform a photography.
FIG. 3 is a circuit block diagram of a camera having an electronic flash equipment according to the earlier technology.
In FIG. 3, there are shown a light quantity indication circuit 10 comprising a CPU 11 and a D/A converter 12, a power supply circuit 50, a light quantity control circuit 30 and a flash light emission circuit 40. The CPU 11 is supplied with a power-supply voltage V.sub.CPU, and the power supply circuit 50 is supplied with a power-supply voltage V.sub.DD.
The CPU 11 feeds to the power supply circuit 50 a power supply signal STVW for activating the light quantity control circuit 30 in a predetermined timing prior to a shutter release, for example, in timing of a shutter button half depression. The CPU 11 performs arithmetic on the quantity of light of emission of the flash light in accordance with an ISO value representative of the photographic speed and an F number representative of the stop or aperture to generate digital light quantity indication data DData, and outputs the same to the D/A converter 12.
The D/A converter 12 performs a D/A conversion on the digital light quantity indication data DData outputted from the CPU 11 to generate an analog light quantity indication signal AIND, and outputs the same to the light quantity control circuit 30.
The power supply circuit 50 comprises: a transistor 21 having an emitter connected to the power-supply voltage V.sub.DD and a collector connected to a power source line (not illustrated) of the light quantity control circuit 30; a resistance 22 disposed between the emitter and a base of the transistor 21; and a resistance 23 disposed between the CPU 11 and the transistor 21. The power supply circuit 50 receives the power supply signal STVW from the CPU 11 to supply to the light quantity control circuit 30 power for operating the light quantity control circuit 30.
The light quantity control circuit 30 comprises a photometric sensor 31, an integration circuit 32 and a comparison circuit 33. The photometric sensor 31 measures reflected light of flash light, which is reflected on a camera subject and is returned. The integration circuit 32 integrates the quantity of light of the reflected light measured by the photometric sensor 31. The comparison circuit 33 compares the integrated quantity of light obtained by the integration circuit 32 with a threshold according to the analog light quantity indication signal AIND generated from the D/A converter 12, and outputs a light emission stop signal STP at the time point when the integrated quantity of light reaches the threshold.
The flash light emission circuit 40 causes the electronic flash to emit light at the time point when a shutter button is entirely depressed, and upon receipt of the light emission stop signal STP, causes light emission of the electronic flash to be terminated.
With reference to FIGS. 3 and 4 there will be explained an operation for a circuit of the camera arranged in this manner.
FIG. 4 is a time chart useful for understanding operation of the circuits shown in FIG. 3.
In FIG. 4, a signal SPI varies to offer an `H` level when a time point that a shutter button is half depressed is given as the starting point, where before the shutter button is half depressed the signal SPI offers an `L` level. Further, before the shutter button is half depressed, the CPU 11 outputs the power supply signal STVW which offers an `H` level. The power supply signal STVW of the `H` level is fed via the resistance 23 to the transistor 21, and thus the transistor 21 is in a turn-off state. Consequently, no electric power is supplied from the power supply circuit 50 to the light quantity control circuit 30 and thereby contributing to the low power dissipation.
Next, in order to take a photograph of a camera subject, a shutter button is subjected to a half-depression state. As a result, a signal SP1 supplied to the CPU 11 is varied in its level from the `L` level to the `H` level. In response to the level change of the signal SP1 to the `H` level, the CPU 11 changes in level the power supply signal STVW from the `H` level to the `L` level. The power supply signal STVW of the `L` level is fed via the resistance 23 to the transistor 21, and thus the transistor 21 is in a turn-on state. Consequently, an electric power according to the voltage V.sub.ST, which is obtained through subtracting a voltage drop due to a turn-on resistance of the transistor 21 from the power-supply voltage V.sub.DD, is supplied to the light quantity control circuit 30.
Further, as mentioned above, the CPU 11 performs arithmetic on the quantity of light of emission of the electronic flash in accordance with the ISO value and the F number to generate digital light quantity indication data DData, and outputs the same to the D/A converter 12. The D/A converter 12 performs a D/A conversion on the digital light quantity indication data DData outputted from the CPU 11 to generate the analog light quantity indication signal AIND, and outputs the same to the comparison circuit 33.
In this condition, the shutter button is entirely depressed. Then, the flash light emanates in accordance with a control of the flash light emission circuit 40. Of the stroboscopic light, a part of reflected light reflected on the camera subject and returned is measured by the photometric sensor 31. The measured quantity of light is integrated by the integration circuit 32 and then fed to the comparison circuit 33. The comparison circuit 33 compares the integrated quantity of light obtained by the integration circuit 32 with a threshold according to the analog light quantity indication signal AIND generated from the D/A converter 12, and outputs the light emission stop signal STP at the time point when the integrated quantity of light reaches the threshold. The flash light emission circuit 40 causes the electronic flash to emit light at the time point when a shutter button is entirely depressed, and upon receipt of the light emission stop signal STP, causes light emission of the electronic flash to be terminated. In this manner, the flash light is controlled to perform a photography for the camera subject.
In the above-mentioned camera according to the earlier technology, there is provided such an arrangement that an electric power is not supplied at ordinary times to the light quantity control circuit for controlling the quantity of light of the flash light emission, but be supplied to the light quantity control circuit at the time point that the shutter button is half depressed, and thereby contributing to a low power dissipation. However, in order to implement the above-mentioned arrangement of the camera, there is a need to increase the output terminals of the CPU by one. This involves complicated processing in the CPU. And in addition, there is a fear that the input and output terminals of the CPU become insufficient. This is caused by a simple idea such that an extra signal (the power supply signal STVW) is added for the purpose of saving power.